The present invention relates to a mold-release agent which may be used in particular in the processing of thermoset moldings. According to the invention, the mold-release agent brings about the ready separation of the molding from the mold, while at the same time bringing about chemical linkage of short-chain and/or long-chain silicone compounds to the surface of the thermoset molding. This achieves effective hydrophobicization of the surface of the molding.
Thermoset materials, in particular epoxy resins, are widely used as electrically insulating materials in medium- and high-voltage technology, for example for the construction of switchgear assemblies. The insulating materials currently known mostly have adequately good electrical properties. The suitability of these insulating materials for use is therefore also based on high requirements for mechanical properties, in particular for mechanical strength and mechanical flexibility, and for behavior in the presence of contaminant layers, and for resistance to the complex electrical and climatic stresses arising at high humidity. Epoxy resins in particular are treated below as a preferred embodiment. Similar considerations apply to other thermoset materials.
Although the properties of epoxy resins are excellent for an insulating material, their use has remained essentially restricted to indoor applications. Insulation materials represent sectors in which epoxy resins are widely used, examples being found in instrument transformers, bushings, power breakers, dry-type transformers, and electrical machinery. Insulation made from epoxy resins is used only rarely in outdoor applications. The causes for this are primarily lack of resistance to UV radiation and behavior in the presence of a contaminant layer. Another aspect here is low resistance to complex electrical and atmospheric stresses. Studies have shown that failure of insulation parts can occur even in indoor systems. If, for example, a deposit of contaminant layers is present on the surface of the molding and there is the possibility of water condensation, some of the stresses which arise are markedly greater than those to which outdoor systems are exposed, and these therefore cause an increased level of failure. Conditions of this type are also termed difficult indoor conditions.
If condensation of water arises on the surfaces under the conditions described, insulating action is substantially dependent on the water-repellency (hydrophobic properties) of the surface. If these surfaces are highly hydrophobic, as is the case with some silicones, deposits of condensed water form as individual droplets separate from one another. However, in the case of hydrophilic surfaces, wetting with a liquid generally gives rise to formation of a coherent film deposit which can lead to short-circuiting of the insulator. A hydrophobic surface is therefore highly advantageous. Epoxy resin surfaces, like many other thermoset resin surfaces, do not have hydrophobic properties, since the epoxy resin matrix has polar character.
Various methods have previously been proposed to improve hydrophobic surface properties. For example, U.S. Pat. No. 4,537,803 claims the addition of a polymerizable silicone oil to the epoxy resin mixture. U.S. Pat. No. 5,306,747 relates to the addition of a modified silicone oil which can react chemically with the resin system. Other proposals likewise include the addition of silicone-based compounds to the epoxy resin mixture. A fundamental disadvantage of all of these methods is that the thermoset matrix to be polymerized is altered by the addition of the silicone compound. For example, the nature of the alterations in physical properties, in reactivity, and in properties relating to flow and to machining may be such as to cause demixing phenomena or electrical discharges in cavities, for example.
It has been found that the process of the present invention eliminates all of the disadvantages mentioned from addition of the silicone compound to the matrix, or entirely prevents the occurrence of these problems, if it is possible to hydrophobicize only the surface of the molding in an adequate manner. Surprisingly, it has now been found that it is possible to hydrophobicize only the surface of the moldings in the molding procedure, without any additional process step, if the mold-release agent used comprises a mixture which comprises (i) at least one compound with mold-release action, preferably a silicone compound, and also (ii) at least one reactive silane compound which is capable of reacting chemically with the epoxy resin surface which forms during the molding process. The mold-release agent of the invention may comprise (iii) other additives where appropriate, for example aqueous and/or non-aqueous solvents. It is surprising that the component (i), i.e. the silicone compound with mold-release action, can be added to component (ii), i.e. the reactive silane compound, without any noticeable inhibition of the mold-release action of the component (i) by the silane compound. At the same time, the reactive component (ii) is capable, during the molding or the mold-release procedure, of reacting in situ with the molding surface which forms. What is formed here is a stable uniform, thin, hydrophobic, silicone-containing layer on the surface of the molding. The surface of the molding is modified chemically in such a way as to have, under difficult indoor conditions, a markedly higher level of hydrophobic properties and a markedly higher level of hydrophobic stability, with the result that surface-modified moldings of this type can be used with advantage as insulating parts in medium- or high-voltage technology. According to the invention, the molding material systems known per se, and also the procedures for their preparation, may be retained here with no alteration. Their physical properties and their behavior in the molding process are not altered. Starting, therefore, from the highly adhesive properties of epoxy resins, the mold-release agent of the invention is used to permit manufacture of moldings of this type without adhesion within the mold, and at the same time to provide effective in-situ hydrophobicization of the surface of the molding formed, in the same process step.
The present invention has been defined in the claims. In particular, the present invention provides a mold-release agent for the production of thermoset moldings with a hydrophobicized surface, characterized in that this comprises (i) at least one compound with mold-release action, preferably a silicone compound, and also (ii) at least one reactive, where appropriate hydrolyzable, silane compound which is capable of reacting chemically with the uncured or incompletely cured thermoset surface. The mold-release agent of the invention may comprise (iii) other additives where appropriate, preferably aqueous and/or non-aqueous solvents.
The term xe2x80x9cuncured or incompletely cured thermoset surfacexe2x80x9d means the surface in situ in the molding process. This term also includes, for example, the surface of a demolded incompletely cured molding which is subjected to post-curing. This means that the mold-release agent of the invention may be used as mold-release agent and hydrophobicizing agent in the molding process and/or as hydrophobicizing agent in the post-curing process, the mentioned advantageous properties being retained.
The present invention also provides the use of the mold-release agent of the invention in the molding process and/or in the post-curing process for the production of thermoset moldings with a hydrophobicized surface.
The invention further provides a process for producing thermoset moldings with a hydrophobicized surface, which comprises using, in the molding process, a mold-release agent which comprises (i) at least one compound with mold-release action, and also (ii) at least one reactive, where appropriate hydrolyzable, silane compound which is capable of reacting chemically in situ with the uncured or incompletely cured thermoset surface.
The present invention also provides the thermoset moldings produced according to the invention.
Mold-release agents for thermoset materials, and also their use and function in the molding process, are known per se. The components used with release action are very often the silicones known per se or polymethylsiloxanes known per se, both of which contain terminal trimethylsilyl groups, or a mixture of compounds of this type. Silicones of this type and mixtures of these may, where appropriate, also comprise silane compounds with mold-release action, such as those defined below as compounds of the general formula (II). The proportion of all of the constituents with release action may be in the range from 1 to 98% by weight, based on the total weight of the mold-release agent. From 11.5 to 15% by weight and from 85 to 98% by weight, based on the total weight of the mold-release agent, have hitherto proven ideal. The remainder of the mold-release agent is composed of the reactive silane compound [component (ii)] defined below, and also of aqueous or non-aqueous solvents and, where appropriate, other additives. A significant factor for the mold-release action of the mold-release agent of the invention is the proportion of preferred polydimethyl-siloxanes of the general formula (I),
(CH3)3Sixe2x80x94Oxe2x80x94[Si(CH3)2xe2x80x94O]mxe2x80x94Si(CH3)3xe2x80x83xe2x80x83(I),
where
m is zero or an integer from 1 to 1 000, preferably an integer from 10 to 50.
The setting points of the compounds of the formula (I) are preferably in the range from xe2x88x9250xc2x0 C. to 0xc2x0 C., and their boiling points are preferably above 80xc2x0 C. ( greater than 80xc2x0 C.), and their molecular weight is preferably in the range from 162 to 74 000.
However, it is not only polydimethylsiloxanes which may be used as compounds with mold-release action. For example, there are also known siloxanes which contain functional groups or else ethyl groups instead of the methyl groups. Compounds of this type also have mold-release action and may be used for the purposes of the present invention together with or instead of compounds of the formula (I). Where appropriate, mixtures of the compounds of the formula (I) may comprise, as mentioned, other silanes with release action, preferably of the general formula (II), as admixture, i.e. at concentrations from 0.01 to 50.0% by weight, preferably from 1.5 to 5.0% by weight, based on the weight of the compounds present of the formula (I). The silanes mentioned preferably have the general formula (II):
(CH3)3Sixe2x80x94Oxe2x80x94[Si(CH3)2xe2x80x94O]mxe2x80x94[SiH(CH3)xe2x80x94O]nxe2x80x94Si(CH3)3xe2x80x83xe2x80x83(II),
where
m is as defied above, and
n is an integer from 1 to 50, preferably an integer from 1 to 10, and the dimethylsilyloxy groups and the methylhydrosilyl-oxy groups in the molecule have been arranged in any desired sequence.
A significant constituent for the purposes of achieving the objects of the invention is the reactive silane compound (also termed xe2x80x9cprimerxe2x80x9d below). Preference is given to compounds of the general formula (III):
(CH3O)3-p(CH3)pSixe2x80x94[Oxe2x80x94Si(CH3)2]sxe2x80x94Rxe2x80x83xe2x80x83(III),
where
p is zero, 1, 2, or 3, preferably zero or 1,
s is zero or an integer from 1 to 100, preferably from zero to 10,
R is a monovalent, unsubstituted or hydroxyl-sub-stituted, organic radical which has one or more reactive groups capable of reacting chemically with an epoxy resin.
In the compound of the formula (III), the methylsilyl group may also, for example, have been replaced by ethylsilyl. Trimethoxysilyl may likewise have been replaced by triethoxysilyl. To this extent, the term methyl also means, in each case independently of the others, ethyl, propyl, or butyl.
The reactive groups mentioned have preferably been selected from glycidyl, hydroxyl, amino (xe2x80x94NH2) and the isocyanate group (xe2x80x94Nxe2x95x90Cxe2x95x90O). Another reactive group which may be used is an ester group, such as an acetyl ester group, which after intermediate hydrolysis gives a compound which can react chemically with the matrix.
R in the compound of the general formula (III) is preferably a 3-glycidoxypropyl radical;
where p is preferably zero or 1, preferably zero, and s is preferably zero. Preference is given to 3-glycidoxy-propyltrimethoxysilane and 3-glycidoxypropylmethyldi-methoxysilane.
If R is a radical which bears a reactive group, this radical preferably has the formula (V):
-A-(reactive group)xe2x80x83xe2x80x83(V),
where
A is a saturated or unsaturated bivalent aliphatic radical which may contain one or more oxygen atoms and/or xe2x80x94NH groups, or is a bivalent cyclo-aliphatic radical, or a bivalent aromatic radical, or a saturated or unsaturated aliphatic radical which may contain one or more oxygen atoms and/or xe2x80x94NH groups and has phenylene substitution, in each case one free valency of this bivalent radical being bonded to a reactive group.
The sated or unsaturated bivalent aliphatic radical A is preferably a radical of the formula xe2x80x94(CH2)txe2x80x94, where t is an integer from 1 to 4, preferably 2, 3, or 4.
The bivalent cycloaliphatic radical A is preferably cyclohexylene, and the bivalent aromatic radical A is preferably phenylene, with preference meta- or para-phenylene.
The saturated or unsaturated aliphatic radical A with phenylene substitution, where in each case one free valency has been occupied by the reactive group, is preferably xe2x80x94(CH2)txe2x80x94(C6H4)xe2x80x94, the reactive group preferably being bonded to the phenylene, preferably in meta- or para-position.
Examples of preferred compounds of the formula (III) which have a radical of the formula (V) are m-aminophenyltrimethoxy-silane [H2Nxe2x80x94(C6H4)xe2x80x94Si(OCH3)3], p-aminophenyltrimethoxy-silane [H2Nxe2x80x94(C6H4)Si(OCH3)3], 3-aminophenoxy)propyl-trimethoxysilane [H2Nxe2x80x94(C6H4)xe2x80x94OCH2CH2Si(OCH3)3], {3-(2-aminoethyl)aminopropyl}tri-methoxysilane [H2NCH2CH2NH(CH2)3Si(OCH3)3].
Examples of reactive compounds of the formula (III) which contain ester groups are acetyloxymethyltrimethoxysilane [CH3COOCH2Si(OCH3)3], and acetyloxypropyltrimethoxysilane [CH3COO(CH2)3Si(OCH3)3].
Examples of reactive compounds of the formula (III) which have a hydroxyl group are hydroxymethyltriethoxy-silane [HOxe2x80x94CH2xe2x80x94Si(C2H5)3] and hydroxymethyltriethyl-silane [HOxe2x80x94CH2xe2x80x94Si(CH3)3].
Isocyanatopropyltriethoxysilane [OCNxe2x80x94(CH2)3xe2x80x94Si(OC2H5)3] is an example of a compound of the formula (III) which contains an isocyanate group.
The compounds of the formula (III), and also their preparation, are known per se and have been described in detail in the literature.
Preference is also given to more complex compounds having two or more reactive groups corresponding to the substituent R and having a greater number of methoxy-silane groups, for example compounds of the general formula (VI):
(CH3O)3-p(CH3)pSixe2x80x94[Oxe2x80x94Si(CH3)2]sxe2x80x94[Oxe2x80x94Si(CH3)(R1)]rxe2x80x94Oxe2x80x94Si(CH3)p(OCH3)3-pxe2x80x83xe2x80x83(VI)
where
r is an integer from 1 to 5, preferably 1, 2, or 3,
R1, independently of one another, are as defined for R above, and
s is as defined above,
where the radicals xe2x80x94Oxe2x80x94Si(CH3)2xe2x80x94 and xe2x80x94Oxe2x80x94Si(CH3)(R1) in the molecule have been arranged in any desired sequence.
To prepare the mold-release agent of the invention, it is preferable to begin by dissolving component (i), i.e. the compound with mold-release action, in a non-aqueous solvent, or by preparing component (i) in the form of an aqueous emulsion. However, the use of organic solvents requires control of all of the relevant risks to the health of the user and to the environment. An emulsion in water is therefore preferable. The aqueous emulsion is prepared in a manner known per se, using emulsifiers. Examples of preferred emulsifiers are organosilyl- or organo-siloxanylcarboxylic acids, and alkylsyloxy-substituted alkylpolyethylene ether compounds.
Components (ii), i.e. the reactive silane compound (III) and/or (VI) is then added to the solution or to the aqueous emulsion, and if water is used it is preferable that the reactive silane compound is likewise emulsified. However, it is also possible for the components in any desired sequence, separately or together, to be dissolved in a solvent, or to be emulsified in water.
The mold-release agent of the invention is composed mainly of the mixture of the dissolved or emulsified component (i) with release action and of the primer, i.e. the reactive component (ii). The mixing ratio of the component (i) with release action to the reactive component (ii) here is important, and may be optimized for the compounds used. For a system based on poly-dimethylsiloxane, where average n is from 10 to 50, a highly active mold-release agent is obtained with a proportion of 1% by weight of primer, based on the total weight of the mixture. The proportion of primer may be reduced as far as 5 parts per thousand, based on the total weight of the mixture. This results in a reduction in the amount of the siloxane chemically bonded to the epoxy resin surface. If the proportion of primer is reduced further, it is no longer certain that there will be uniform and coherent binding of the siloxanes to the epoxy resin surface. If the proportion of primer is increased, the release action generated by the component (i) with release action with respect to the mold wall reduces. The proportion of primer, or the proportion of reactive silane of component (ii), is therefore preferably from 5 parts per thousand to 5% by weight, preferably from 0.1 to 2% by weight, based on the total weight of the mold-release agent of the invention.
In the case of water-containing mold-release agents, when the primer is added there is at least some hydrolysis of the methoxy groups present in component (ii). For solvent-containing and substantially water-free mold-release agents, it is mainly atmospheric moisture which hydrolyzes the methoxy groups. The hydrolysis of all of the methoxy groups, or complete crosslinking, generally takes place by virtue of atmospheric moisture after application to the surface of the molding.
A release agent based on an oil-in-water emulsion generally requires a preheated mold for processing. However, this requirement is generally complied with in most cases in the processing of casting resins. To increase emulsifiability, a surfactant or a mixture of surfactants may be added to the mold-release agent. The proportion of the surfactant is preferably in the range from 0.002 to 5% by weight, preferably from 0.05 to 2% by weight, based on the total weight of the mold-release agent of the invention.
According to the invention, the composition used capable of thermoset curing may be a thermoset molding composition known per se. A large number of thermoset plastics is known from the literature. The extent to which these are present in the molding is at least that to which these thermoset compositions are capable of giving effective chemical reaction with the reactive silane compound [component (ii)]. Examples of thermoset molding compositions are polycondensates and polyadducts. Examples of polycondensates are curable phenol-formaldehyde plastics, curable bisphenol resins, curable urea-formaldehyde plastics, polyimides, poly-benzimidazoles. Examples of thermoset plastics in the form of polyadducts are epoxy resins, unsaturated polyester resin molding compositions, DAP resins (poly-diallyl phthalate), curable melamine-phenol-formaldehyde molding compositions, and crosslinked polyurethanes. Preference is given to those with good electrical properties, preferably aromatic or cyclo-aliphatic epoxy resins, and also PU casting compositions, in particular, as are known per se and used in the electrical industry, filled epoxy resins and PU resins. Preferred fillers which may be used are powdered quartz, aluminum oxide, and dolomite in a variety of ground forms known per se, and also those with silanized surfaces.
Examples of additives which may be used for thermoset compositions in the form of poplycondensates or of polyadducts, besides the resin/hardener/accelerator system for thermosets, are release agents, lubricants, fillers, pigments, coupling agents, stabilizers, and inhibitors. These compounds are known per se.
When the mold-release agent is used, it is applied by means of methods known per se to the heated mold wall. Examples of these methods are spraying, brushing, wiping, and/or dipping. The mold may be composed of a variety of metals, preferably of steel, copper, or aluminum, or else of silicate glasses or ceramics. The mold is preferably composed of a material based on steel or aluminum, or of polymeric materials, such as filled epoxy resins. The surface roughness of the mold wall has been observed to be an important process pararmeter. The surfaces should have the lowest possible roughness, preferably for example that which is obtained when steel is honed or lapped. The temperature of the mold wall may be from 20 to 180xc2x0 C. The temperatures are preferably set at from 60 to 140xc2x0 C. Once the mold-release agent of the invention has been applied, the solvent evaporates, and the mold-release agent film composed of the formulation of the invention remains on the surface of the mold. When, using known processes, such as vacuum casting, pressure gelation, or open casting, the casting resin composition is cast into the mold thus prepared, the exothermic curing reaction of the thermoset molding composition starts from the mold wall, since the heated mold already has the activation energy needed. In this process, the reactive component (ii) is simultaneously incorporated chemically into the matrix of the epoxy resin or onto its surface by way of the radical R defined above and, where appropriate, also by way of the methoxysilane group. This ensures that the entire surface receives a hydrophobicizing impregnation during the curing reaction. It is also possible here to bond compounds of the formula (II) chemically, directly or indirectly, to the surface of the cured molding which forms. During the curing reaction, the mold may be externally heated, or else cooled. After the gelling process, the moldings may be removed from the mold and cured completely in a further step. To this extent, it is also possible that the surface of an incompletely cured demolded molding which is subjected to post-curing is treated in advance with the mold-release agent of the invention. Any desired mold-release agent may be used here in the molding process. The temperatures used in the post-curing process here are usual and known per se, and are comparable with the molding temperatures.
The surfaces produced by the process described above exhibit hydrophobic properties, as are known for surfaces comprising silicones. There is a marked improvement both in hydrophobic properties and in hydrophobic stability. It is moreover possible to measure some recuperation of hydrophobic properties. However, it is apparent here that since the layer of the siloxanes is thin, the recuperation found with full-body silicone cannot reliably be provided. The hydrophobic properties have also proven to be unsuitable for instances where the surface has exposure to an abrasive process, for example where the surface is ground away by sandstorms. The application sector for the mold-release agent of the invention and for the moldings produced according to the invention is therefore mainly in the construction of switchgear, where difficult indoor conditions have to be withstood for long periods. Other usable application sectors are insulating parts for high-voltage switches, and outdoor insulation in a maritime atmosphere or where there is industrial pollution. Another significant factor for the invention is that all of the mechanical properties and the non-surface-dependent electrical properties of the thermosets, for example of the epoxy resin, are retained unaltered by the modification. The examples below illustrate the invention.